A gas measuring apparatus which is equipped with a gas sensor exposed to exhaust gas from an automotive vehicle and a calculating portion which uses an output of the gas sensor to calculate the concentration of NOx contained in the exhaust gas (see Patent Literature 1, as listed below).
The gas sensor is equipped with a solid electrolyte body which has oxygen ion conductivity and a plurality of electrodes formed on both surfaces of the solid electrolyte body. The solid electrolyte body and the electrodes form three cells: a pump cell, a monitor cell, and a sensor cell. The pump cell is a cell which reduces the concentration of gas. The monitor cell is a cell which detects the concentration of oxygen slightly remaining in the gas whose concentration of oxygen has been reduced by the pump cell. The sensor cell is a cell which measures the sum of concentrations of oxygen and a given gas component (NOx) contained in the gas whose concentration of oxygen has been reduced by the pump cell.
An amount of electric current (i.e., a monitor cell current Im) which corresponds to the concentration of oxygen remaining in the gas flows through the monitor cell. An electric current (i.e., a sensor cell current Is) also flows through the sensor cell. The sensor cell is active both to oxygen and to the given gas component. The sensor cell current Is, therefore, includes a component (i.e., an oxygen dependent current Iso) arising from the concentration of oxygen and a component (i.e., a given gas dependent current Ix) arising from the concentration of the given gas component. The accurate determination of the concentration of the given gas component required elimination of the oxygen dependent current Iso from the sensor cell current Is to precisely derive the given gas dependent current Ix.
It is, however, usually impossible to directly measure the oxygen dependent current Iso. The above gas concentration measuring apparatus, therefore, uses the monitor cell current Im instead of the oxygen dependent current Iso. Specifically, the monitor cell current Im, like the oxygen dependent current Iso, has a correlation with the concentration of oxygen. The monitor cell current Im is, thus, subtracted from the sensor cell current Is(=Iso+Ix) to obtain an approximate value of the given gas dependent current Ix. The approximate value is used to calculate the concentration of the given gas component. This operation is made by the above calculating portion.